Ok then. I have junk in my doors and it's quiet. The bass out of the Alpines in the front doors are punchier and more solid than before. f**k it, I'm happy, and I'm a fussy mother f**k...
BTW, as stone said, it may be that you need 2 layers, but what I did was line the inner main skin of the door, and the outer door surface behind the door trim, essentially 2 layers I suppose.

Sorry, I didn't mean to say it was junk... Just in comparison to proper deadener, it doesn't really compare.

In my old Commodore I put two layers on the outer door skin then two layers of paint on stuff. Then two more layers of Flashtac on the inner door skin, where the speaker is mounted. It did improve the performance but I ended up spending alot on that amount of Flashtac anyway.

As it has been said, it depends on your application - reducing road noise or making your speakers sound better (or both!).

I used Stinger Roadkill for my front doors - it works very well and sticks on with ease. This was all in the name of reducing resonance within the door.

The next thing I did was cut out 3mm MDF panel pieces and cover up all the access holes in the skeletal part of the door to create more of a 'sealed' door for my speakers to take advantage of.

I found the combined result was fantastic...I am fairly fussy and will admit, some fine tuning was / is definitely required...but it's a good start in the right direction IMHO.

Stinger Roadkill (4 door expert kit) - RRP $178, paid $130 or something from JB Hi-fi.

I've said it before and I'll say it again, you are not doing your speakers justice until you have sound deadened !